Such an ignition coil is described in DE 100 14 115. The conventional ignition coil has an inner magnet core made up of lamellar sheet-metal strips which are stacked on top of each other. The sheet-metal strips form an overall rectangular cross-section surface. The inner magnet core is concentrically surrounded by a primary bobbin and a secondary bobbin. The shape of both the primary bobbin and the secondary bobbin is adapted to the cross-section shape of the inner magnet core, the primary bobbin and the secondary bobbin each having rounded edges along the corner areas of the inner magnet core. In addition, the spaces between the inner magnet core, the primary coil with its primary bobbin, and the secondary coil with its secondary bobbin are surrounded by an insulation compound, in particular by an insulation resin which is used for electrical insulation between the voltage-carrying components.
During manufacture of the primary coil and the secondary coil, the bobbin will be provided with a winding of the primary wire and the secondary wire, respectively. This is carried out in that the primary bobbin and the secondary bobbin are rotatably supported in their symmetry axes and, during the rotation, pull a wire off a supply spool and the appropriate windings are applied to the primary bobbin and the secondary bobbin. The geometric design of the primary bobbin and the secondary bobbin with its essentially rectangular cross-section surface having rounded edges results in different pull-off speeds of the wire during the rotation of the primary and the secondary bobbins depending on the angular position of the bobbins according to FIG. 6, curve A. This has the effect that the highest wire pull-off speeds prevail in the corner areas of the primary bobbin and the secondary bobbin, resulting in the primary wire and the secondary wire being applied to the primary bobbin and the secondary bobbin under relatively high tension. This causes a compaction of the wire layers in the corner areas of the bobbins, which makes the subsequent impregnation or insulation of the primary coil and the secondary coil with the insulation resin more difficult, since the resin is unable to properly fill the spaces between the individual wire layers. The electrical insulation capability and the breakdown capability of the ignition coil are reduced in the corner areas.
In so-called rod ignition coils, i.e., ignition coils whose coils are directly situated in a borehole of the cylinder head of the internal combustion engine, it is conventional to provide a circular cross section of the inner magnet core (EP 0 859 383). In this case, sheet-metal strips having different widths are used for the inner magnet core to make the circular cross section possible.
Furthermore, it is described in DE 299 01 095 to provide an inner magnet core in a rod ignition coil which has a substantially rectangular cross section. Only the lowermost and the uppermost strips of the sheet-metal packet each have a reduced width, the width being approximately one third to one half of the width of the remaining sheet-metal strips. This makes it possible, according to DE 299 01 095, to achieve a cross section adapted to a circular cross section. The disadvantage is that the cross-section surface of the magnet core (compared to a rectangular cross section) is reduced and the magnetic properties of the sheet-metal packet are not optimally utilized. Moreover, the problems in the corner areas of the magnet core having the increased wire pull-off speeds and the associated disadvantageous effects during winding of the primary bobbin and the secondary bobbin remain.